The abundance of cattle is the primary influence on the
prevalence of two tick-borne pathogens, according to a paper in the April Applied and Environmental Microbiology.
One of these, Anaplasma phagocytophilum,
causes human granulocytic anaplasmosis, and the other, Borrelia burgdorferi, causes Lyme disease. Although other studies
have examined the effect of hosts on tick and tick-borne pathogen dynamics,
this is the first to clarify the role of host abundance on prevalence of the
two pathogens in their natural habitat, where wildlife and domestic livestock
coexist.

The impetus for the research was the fact that in recent
decades, gamekeepers in the study area, a wildlife preserve in the northern
Iberian peninsula, had suffered Lyme disease, and had noticed an increasing
abundance of ticks, says first author Francisco Ruiz-Fons, of the Instituto de
Investigación en Recursos Cinegéticos, Ciudad Real, Spain. “Our working
hypothesis was that wild and domestic ungulates would be primary drivers of the
abundance of Ixodes ricinus ticks,
and of the prevalence of pathogens transmitted by this tick species, in natural
foci where they coexist and where B.
burgdorferi and A.phagocytophilum
are endemic.”

That hypothesis held up, but somewhat differently from
expected, having opposite effects on A.
phagocytophilum and lyme bacterium, B. burgdorferi, in reducing prevalence
of the latter. That seemingly counterintuitive finding stemmed from the fact
that “cattle and wild ungulates may act as diluters of borrelias by diverting
infected ticks’ bites from competent reservoirs such as birds or small
mammals,” says Ruiz-Fons. Thus, more cattle meant reduced numbers of Lyme
disease bacteria.

“The most important application of our findings is that if
we want to reduce the risk of animals and humans becoming infected by
[tick-borne] pathogens we should control the infestation by ticks in cattle—and
perhaps in wildlife—rather than by reducing cattle or wild host abundance in
areas where wild and domestic animals coexist,” says Ruiz-Fons.